Andy Gill wrote:
On Oct 25, 2006, at 2:01 AM, Simon Marlow wrote:
The use of Notes in Core is a slight concern - the simplifier will
almost certainly rearrange code without paying attention to the
Notes, indeed it looks like you have some hacks already to avoid this
(the extra cases in exprArity look quite strange). Weren't you using
explicit expressions before? Why the change of design?
Compile time and less hacks. We use Note for SCC right now, so this
seemed like the right place to put it. The exprArity
hack is there because of the code
\ x -> _tick \ y -> expr really does have aritty 1; it will
translate into
\x -> case tickme 0 of
_-> \ y -> expr
We should choose the design that is as modular as possible; that is, requires
the fewest changes elsewhere. The example above indicates to me that using a
Note is perhaps not the best design, because most of the simplifier assumes that
the semantics of 'Note e' are the same as 'e', but this breaks that assumption.
Indeed, you already needed to modify various predicates in the simplifier to
do something special for Tick expressions.
Essentially you've added an extra constructor to Expr but hidden it inside Note.
I suspect we'll be finding parts of the simplifier and other core-to-core
passes that need to be modified for a long time.
I realise that SCC already breaks the rules a bit, and that's unfortunate too.
But it doesn't seem as invasive as the Tick annotations.
The new design has less backend hacks in CorePrep and CoreToStg.
Ok, can you explain why that is?
Please don't take this as a criticism - HPC is a fantastic addition to GHC. I'd
just like to explore the design and see if there are any ways we could improve
it, and hopefully give ourselves fewer headaches in the future.
Cheers,
Simon
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